Rigid polymeric materials contain little or no plasticizer and commonly are used in a wide variety of uses sheltered from direct exposure to outside sunlight, heat and UV radiation. External exposure to heat and sunlight is known to degrade various polymeric materials and invariably break down the polymeric chemical structure and otherwise deteriorate the polymeric materials and physical appearance of plastic parts. Polymeric structural plastic substrates are engineered rigid plastics having good mechanical strength and dimensional stability suitably designed for external housing and other outdoor uses. Polymeric substrates frequently contain little or no plasticizer and typically have a good balance of physical properties such as high tensile, stiffness, high compressive and shear strength, and high impact properties. However, if outdoor products made from these polymeric substrates are to have any practical commercial value in outdoor exposures, such as house siding, window and door frames and profiles, exterior sash applications, shutters, roof vents, outdoor fences and decking, and similar external structures, the polymeric materials must have resistance to outdoor exposure environments, especially sunlight and heat.
Weathering deterioration of such polymeric materials is often initiated by damaging radiation from exposure to the sun or by hydrolysis of esters, carbonates, or amides in the polymeric structures. For instance, Arizona exposure is sunny and dry which emphasizes UV exposure, while Ohio exposure is less sunny, wetter and subject to more acid rain, which emphasizes hydrolysis deterioration. Sun energy penetrating the atmosphere has been measured as a function of wavelength. Vacuum ultraviolet and ultraviolet rays shorter than 2800 angstrom units are absorbed by the atmosphere and consequently the earth is shielded from severe ultraviolet damage from these wavelengths. However, some of the ultraviolet energy, most visible energy, and much infrared energy penetrates the atmosphere and can cause considerable damage upon exposure. For instance, vacuum ultraviolet causes chemical damage and can be of particular concern to polymeric applications in space where little atmosphere is available for protection. The infrared region of sunlight energizes molecular vibration including stretching, bending, rotating and heating of materials without much chemical damage. However, ultraviolet energies absorbed by polymeric materials are capable of breaking chemical bonds leading to oxidation and polymer chain scission, which often causes discoloration and imbrittlement of plastic materials. Breakage of polymeric bonds resulting in reduction of molecular weight causes brittleness. Therefore, loss of impact strength is often indicative of the degradation which has occurred. Some polymers containing ester, amide, or carbonate groups lose molecular weight by hydrolysis and become brittle in moist atmospheres, particularly in atmospheres high in acid rain.
In the past, some rigid PVC products used for exterior applications consisted of a structural substrate of rigid PVC and a top thinner surface layer of plasticized PVC called a cap or capstock. The layered composites are comprised of a thin layer of semi-rigid PVC co-extruded or laminated over a PVC substrate to achieve good weatherability and color retention and protect the rigid structural substrate understructure. Semi-rigid and flexible PVCs contain high levels of plasticizer which tend to resist heat and sun degradation. However, the difference in glass transition in conjunction with stiffness and expansion differences of the semi-rigid PVC cap relative to the rigid PVC substrate causes a differential expansion and contraction, and, therefore, warping and distortion in the heating and cooling steps, especially in extrusion processes, where extrudates of semi-rigid PVC cap over rigid PVC substrate exhibit dimensional distortion, especially during cooling. In addition, semi-rigid capping layers provide limited color choices, mostly limited to opaque colors only, and are not useful for bright masstone colors. For instance, U.S. Pat. No. 4,100,325 teaches PVC surface capstock containing high levels of plasticizer to provide useful layered composites exhibiting considerably improved appearance retention and degradation resistance while maintaining structural integrity and impact resistance. Although effective to reduce degradation, co-extrusion of plasticized PVCs capstock in conjunction with rigid PVC substrates in wide panels sometimes can result in distortion and warping during the co-extrusion process due to differences in Tg, stiffness, thermal expansion and contraction of the two layers from the wide heat differentials encountered in heating and cooling steps of the co-extrusion process.
In the unrelated art of flexible packaging, glycol modified polyethylene terephthalate known as PETG is suggested in U.S. Pat. No. 4,505,400 as a sealing film layer combined with a polyester film layer subjected to molecular orientated structuring to provide flexible films useful in flexible film packaging. Similarly, U.S. Pat. No. 6,068,900 discloses a plastic container having an outer layer of PETG and an inner layer of polyethylene terephthalate and/or polyethylene naphthalate.